Cathode-ray tubes of the focus-mask variety



Oct. 24, 1961 R. H. GODFREY CATHODE-RAY TUBES OF THE FOCUS-MASK VARIETY Filed Jan. 27, 1958 2 Sheets-Sheet 1 m m m m RICHARD H. E EDI-REY 42%! 2416/04 A TToAA EY Oct. 24, 1961 Filed Jan.

R. H. GODFREY CATHODE-RAY TUBES OF THE FOCUS-MASK VARIETY 2 sneeis-sneet 2 Rn: HARD HE DDFREY 3,905,921 CATHSDE-RAY TUBES OF m FOCUS-MASK VARIETY Richard H. Godfrey, Landisvilie, Pa, assignor to Radio This invention relates to improvements in color-kinescopes and other cathode-ray tubes of the kind wherein the beam-electrons are subjected toa focusing action by elemental mask-aperture lenses immediately prior to im- :1 j

pinging upon the screen or target of the tube. Such cathode-ray tubes are herein referred to as being of the focus-mask variety.

If the screen and the envelope walls of a focus-mask tube are operated at the same high potential and the mask is operated at a lower potential, the field between the tubes plane-of-deflection and the mask deceierates the beam-electrons, and the field between the mask and the screen accelerates the beam-electrons. Consequently, the electrons before reaching the mask are refracted away from the tube-axis and, after passing through the maskapertures, are refracted toward the tube-axis. This refraction phenomenon may be considered to be separate from the focusing effect produced by the mask-apertures acting as lenses. from the tube-axis as they approach the mask is beneficial in that it reduces the scanning power required of the yoke. However, unless the outward refraction effect before the mask is exactly off-set or balanced (to achieve zero net refraction) by the refraction in the other (i.e. inward) direction, between the mask and screen, a phosphor dot (or line) screen prepared by optical projection in a lighthouse (as used in the manufacture of mosaicscreens for use in shadow-mask tubes) cannot be employed since the terminal points of the light paths and the electron paths on the phosphor-screen will not coincide and misregister exists.

One way of achieving the register, or coincidence of the optical and electron terminals in a tube of the focus-mask variety is to employ'an optical-lens in the screen-plotting operation. (As to this see Epstein et al. 2,817,276; see also copending application Serial No. 585,- 254, which is now U. S. Patent No. 2,885,935, to the same inventors.) Another way of achieving such coincidence is by balanced refraction which involves the provision of a number of electrically adjustable fieldforming electrodes in the cone of the tube-as taught by this inventor in Serial No. 711,249, filed concurrently herewith. While both the optical method and the electrical (field-forming) method operate quite satisfactorlly, it is extremely diflicult as a practical matter to achieve coincidence of optical and electrical terminals at the extreme marginal edges of the screen by either method. That is to say focus-mask tubes embodying such methods may exhibit color-dilution or analogous imagedefects within a very narrow band or halo at the edge of the screen. Although such a halo can be made invisible to the spectator either by concealing it behind a bezel or escutcheon, or by reducing the dimensions of the raster, such solutions of the problem are obviously undesirable.

Accordingly, the principal object of the present invention is to provide an improved focus-mask tube and one characterized by its highly efiicient use of the available screen-area and by its substantial freedom from imagedefects, even adjacent to the edges'of the screen.

The foregoing and related objects are achieved in accordance with the invention by the provision of electrode means disposed about the marginal edge or edges of the Refraction of the beam-electrons away 3,905,921 Patented Oct. 24., 1961 mask and extending a limited distance into the focusingfield within the mask-screen space. This electrode means, when utilized with either previously mentioned optical or electrical methods for achieving register, serves to reshape or to tilt the adjacent equipotential surfaces in the direction and to the degree required to provide zero net refraction and hence zero misregister in the region of the edges of the screen.

The invention is described in greater detail in connection with the accompanying two sheets of drawings, wherein:

FIG. 1 is an elevational view, partly in section, of a cathode-ray tube of the focus-mask dot-screen variety and which has been provided, in accordance with the invention, with a field shaper surrounding the maskand extending a limited distance into the mask-to-screen space;

FIG. 2 is a fragmentary view, taken from the gun side of the screen-unit of the tube of FIG. 1, showing the hexagonal mosaic patterns of the mask-apertures and color-phosphor screen-areas;

FIG. 3 is an enlarged fragmentary view of a portion of the front-end of the tube of FIG. 1 showing the fieldshaper of the invention; the drawing being marked with equipotential lines indicative of the accelerating and focusing field in the mask-to-screen space and showing how the field-former operates to til said lines in the direction I and to the extent required to provide substantially zero net refraction of the beam-electrons which pass through that part of said accelerating field;

FIG. 4 is a chart showing misregister (of the beamspots with respect to the phosphor screen dots) as a function of the height of the field shaper; and

FIGS. 5 and 6 are views similar to FIG. 3 but showing alternative forms of field-shaping electrodes within the purview of the invention. 7

As previously set forth, when the beam-electrons in a tube of the focus-mask variety such, for example, as the 3-gun tri-color kinescope 1 of FIG. 1, pass through an accelerating field in their transit through the space q between the mask 3 and screen 5, the beams (exemplified by the red beam r) are subjected not only to a focusing action but to inward refraction as well. The inward refraction or bending of the beams is especially pronounced adjacent to the edges of the screen, where the beams are at the limit of their scanning movements. Excessive refraction in this region of the screenunit may be attributed to the fact that the terminal portions of the equipotential surfaces (e FIG. 3), of which the accelerating field is comprised, are funneled or crowded into the space (s FIG. 3) between the walls of the tube and the edge of the mask. In any event, ex-

' cessive refraction, and consequent picture-defects at the adjacent edge portion of the screen are minimized, in accordance with the invention, by the provision of an auxiliary electrode or field-shaper" 7 (see FIGS. land 3) disposed adjacent to the edge of the mask 3 and extending a limited distance into the focusing field e in the space q between the mask 3 and screen 5. This fieldshaping electrode 7 operates to tilt the equipotential surfaces e, in that region of the tube, in the direction and to the extent (hereinafter specified) required to provide zero net refraction, and hence zero misre'gister of the electron-beam spots with respect to the phosphor screen dots. Thus, when used in combination with previously mentioned electrical and light optical methods, substantially zero misregister can be achieved over the entire screen. V Y

In the embodiments of the invention which have been selected for illustration (wherein the mask 3 and screen 5 are circular in shape), the auxiliary electrode or fieldshaper comprises a metal ring or annulus 7 surrounding the mask and electrically and mechanically connected thereto. Thus, referring particularly to FIG. 3, wherein the peripheral edge of the thin-metal mask 3 is attached (as by Welding) to the outer surface of a conventional (L-shape) rigid metal frame 9, the field-shaping electrode is shown to comprise an annular metal strip 7 welded to the outer surface of the rearwardly bent edge portion of the mask. In another embodiment of the invention, shown in FIG. 5, wherein the thin-metal mask 3 is attached to the inner surface of the rigid metal frame 9 the projecting portion of the long arm 9a of the L- shape frame comprises the field-shaper. In the case of a frameless mask, referring now to FIG. 6, the fieldshaper may comprise an embossment 3b surrounding the apertured area of the mask. In all these embodiments the field-shaper preferably terminates in a rounded edge (which may take the form of a bead 7b FIG. 3) to minimize the possibility of arc-over between it and the (big potential) metallized surface m of the screen 5.

The optimum height of the field-shaping electrode (i.e., the distance it projects into the electric field e between the mask and the screen) may be determined by the observed misregister in a tube of the same type and dimensions operated without a field-shaper. As to this, reference is now made to FIG. 4 which charts register change as a function of the height of the field-shaping electrode 7 in a focus-mask color-kinescope of a type similar to the one shown in FIG. 1 (i.e. one provided with field-forming electrodes 11, 13 and 15 in the cone of the tube) and wherein the q (i.e. the mask-to-screen spacing) was approximately 0.535 near the edge of the screenunit. In such a tube, operating without the field-shaping electrode 7 of the present invention, the register curve A exhibited an inflection or hook A (i.e. a sudden change in direction) near the edge of the screen. Now, as previously mentioned, to correct for this hook by means of an optical-lens (while laying down the colorphosphors) would be extremely difl'icult because of the Sharp changes that would be required in the slope of the lens near its edge. (In fact, the light transmitted through such a lighthouse lens at this radius might be insuflicient to provide the proper photographic exposure). By the addition of a field-shaper or dehooking band (as it is designated in the chart of FIG. 4), the register-vs. screen-radius curve can be shaped to permit a lighthouse lens of a contour easy to attain and to use. The latter is important as Well since, if the register correction is all in the same direction, lens off-set over the lighthouse source can be used to correct at least in part for dot-trio deformation (called crowding) brought about by the curved screen-and mask assembly.

Tubes were operated with dehooking bands of various heights, two of which are indicated on the register curves B and C. The preferred height in this case was .100 (as indicated on the curve B) since this height eliminates the .hook or inflection (shown in curve A) and, at the same time, provides a relatively low inward radial register change. A dehooking band more than .100" high, although eliminating the hook (of curve A) gave rise to a somewhat higher inward radial register change, as shown for example by curve C.

As previously brought out, optimum freedom from color-impurities and other image-defects is achieved over the entire area of the screen, including its edge portion, when the invention is applied to a tube containing a series of field-forming electrodes (11, 13, 15 FIG. 1) through which the electron-beams pass in their transit to the mask 3. As described in this inventors copending disclosure (Serial No. 711,249) these auxiliary electrodes 11, 13 and 15, when appropriately energized (e.g. in the manner indicated in FIG. 1) operate to alter the contour of the equipotential surfaces (of which the decelerating electrostatic field in the space between the tubes planeof-deflection P-P, and the mask 3, is comprised) in such a way that the electron-beams (from the three guns 17r, 17b and 17g) are refracted outwardly only a suificient amount along curved paths (indicated by the path of the red beam r) to compensate for the inward refraction between the mask 3 and screen 5. Thus, a straight line AA (FIG. 1) drawn from the point of electron-impact (on any phosphor dot R, B or G) on the screen 5 through the center of the appropriate mask-aperture 3a (FIG. 2) can be extended rearwardly to the color-center (r') of that beam in the tubes plane-of-defiection PP.

It should now be apparent that the present invention provides a focus-mask tube characterized by a highly efiicient use of the available screen-area and by its substantial freedom from image defects even at the edge of the screen.

What is claimed is:

1. In a color-kinescope of the kind containing a beam source of electrons and an apertured mask through which beam electrons from said source pass in their transit to individual ones of the elementary areas of a color-screen of the mosaic variety and wherein said beam electrons are subjected (a) to outward refraction by reason of the presence of a decelerating electric field in the space between said source and said mask and (b) to inward refraction by reason of the presence of an accelerating focusing field in the space between said mask and said mosaic screen, the improvement which comprises: electrode means disposed adjacent to a marginal edge portion of said mask and electrically associated therewith for alterin the degree of inward refraction to which said beam electrons are subject in the adjacent marginal portion of said mask-to-screen space to correspond substantially with that degree of outward refraction to which said beamelectrons are subject prior to entering said last mentioned space.

2. The invention as set forth in claim 1 and wherein terminal means are provided for subjecting said mask and said electrode means to the same operating potential.

3. The invention as set forth in claim 1 and wherein said electrode means comprises a support for said mask.

4. The invention as set forth in claim 1 and wherein said electrode means comprises an integral marginal edge portion of said mask bent outwardly in the direction of said mosaic screen.

5. In a color-kinescope of the kind containing a beam source of electrons and an apertured mask through which beam-electrons from said source pass in their transit to individual ones of the elementary areas of a color-screen of the mosaic variety and wherein said beam electrons subjected both to (a) outward refraction by reason of the presence of a decelerating electric field in the space between said source and said mask and to (b) inward refraction by reason of the presence of an accelerating focusing field in the space between said mask and said mosaic screen, the improvement which comprises: electrode means disposed adjacent to the Outer boundary of said mask and extending therefrom a limited distance in the direction of said mosaic-screen and adapted electrically to tilt the lines of force of which the adjacent part of said accelerating field is comprised in the direction and to the extent required to provide substantially zero net refrac tion of the beam-electrons which pass through said part of said accelerating field in their transit to the elementary areas of the mosaic which lie adjacent to the outer boundaries of said screen.

References Cited in the file of this patent UNITED STATES PATENTS 2,728,782 Smith Dec. 27, 1955 2,793,319 Nunan May 21, 1957 2,824,988 Cone Feb. 25, 1958 2,914,691 Evans Nov. 24, 1959 

